-LRB- CNN -RRB- -- An appellate court on Thursday ordered a new trial for a Florida woman who was sentenced to 20 years in prison for firing a gun to scare off her allegedly abusive husband , ruling that a jury was improperly instructed on self-defense .

Marissa Alexander 's case will be retried because the jury was wrongly told that -- for her to claim self-defense -- she needed to prove beyond a reasonable doubt that her husband was about to seriously harm her , the appellate court said .

Rather , the appellate court pointed out , the prosecution had the burden to prove that Alexander herself was guilty of aggravated assault .

`` Because the jury instructions on self-defense were fundamental error , we reverse '' the conviction , a three-judge appellate panel said .

A jury convicted Alexander , 31 , of aggravated assault in March 2012 after just 12 minutes of deliberation .

Alexander 's lawyer said he told her about the new trial by phone .

`` Marissa was ecstatic and obviously she 's incredibly thankful and wants to get back with her family , '' defense attorney Bruce Zimet told CNN .

` Stand your ground ' denied in Alexander case

The case gained the attention of civil rights leaders , who say the African-American mother of three was persecuted because of her race , and from others , including U.S. Rep. Corrine Brown , who said the sentence was too harsh and Alexander never should have been charged .

Alexander claimed self-defense , saying she was attempting to flee her husband , Rico Gray , on August 1 , 2010 , when she picked up a handgun and fired a shot into a wall .

She said her husband had read cell phone text messages that she had written to her ex-husband , got angry and tried to strangle her .

State Attorney Angela Corey had said the case deserved to be prosecuted because Alexander fired in the direction of a room where two children were standing .

Corey had said she offered Alexander a plea bargain that would have resulted in a three-year prison sentence , but Alexander chose to take the case to a jury trial , where a conviction would carry a mandatory sentence under a Florida law known as `` 10-20-life . ''

The law mandates increased penalties for some felonies , including aggravated assault , in which a gun is carried or used .

Opinion : Shame of mandatory minimums

During sentencing in May 2012 , a judge said he had no choice but to sentence her to 20 years .

`` Under the state 's 10-20-life law , a conviction for aggravated assault where a firearm has been discharged carries a minimum and maximum sentence of 20 years without regarding to any extenuating or mitigating circumstances that may be present , such as those in this case , '' Judge James Daniel said that year .

Before the trial , Alexander unsuccessfully tried to use Florida 's Stand Your Ground law to argue she was immune from prosecution . To win the immunity hearing , she would have needed to show that she was more likely entitled than not to use force .

But a judge in the pretrial immunity hearing rejected the request , saying Alexander 's decision to go back into the house was not consistent with someone in fear for her safety .

In Thursday 's decision , the appellate court said Alexander would not get a new immunity hearing . But she can still claim self-defense at trial .

The office of the Florida state attorney for Duval County said Thursday that Alexander 's conviction `` was reversed on a legal technicality . ''

`` We are gratified that the court affirmed the defendant 's Stand Your Ground ruling '' denying pretrial immunity , the office said .

Florida lawmakers wo n't have special session on ` stand your ground '

InSession 's Jessica Thill and CNN 's Chuck Hadad , Michael Pearson and Marylynn Ryan contributed to this report .

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NEW : Lawyer : Marissa Alexander is `` ecstatic '' and `` incredibly grateful ''

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Court : The jury received bad self-defense instructions

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Alexander was convicted in March of aggravated assault

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Alexander said she fired a warning shot to fend off her allegedly abusive husband